[PATCH 2/2] futex: Use atomic64_try_cmpxchg_relaxed() in get_inode_sequence_number()

[Uros Bizjak]

Optimize get_inode_sequence_number() to use simpler and faster:

  !atomic64_try_cmpxchg_relaxed(*ptr, &old, new)

instead of:

  atomic64_cmpxchg relaxed(*ptr, old, new) != old

The x86 CMPXCHG instruction returns success in ZF flag, so
this change saves a compare after cmpxchg. The generated
code improves from:

 3da:	31 c0                	xor    %eax,%eax
 3dc:	f0 48 0f b1 8a 38 01 	lock cmpxchg %rcx,0x138(%rdx)
 3e3:	00 00
 3e5:	48 85 c0             	test   %rax,%rax
 3e8:	48 0f 44 c1          	cmove  %rcx,%rax

to:

 3da:	31 c0                	xor    %eax,%eax
 3dc:	f0 48 0f b1 8a 38 01 	lock cmpxchg %rcx,0x138(%rdx)
 3e3:	00 00
 3e5:	48 0f 44 c1          	cmove  %rcx,%rax

Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Darren Hart <dvhart@xxxxxxxxxxxxx>
Cc: Davidlohr Bueso <dave@xxxxxxxxxxxx>
Cc: "André Almeida" <andrealmeid@xxxxxxxxxx>
---
 kernel/futex/core.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/futex/core.c b/kernel/futex/core.c
index 3146730e55f7..11795439efb7 100644
--- a/kernel/futex/core.c
+++ b/kernel/futex/core.c
@@ -185,8 +185,9 @@ static u64 get_inode_sequence_number(struct inode *inode)
 		if (WARN_ON_ONCE(!new))
 			continue;
 
-		old = atomic64_cmpxchg_relaxed(&inode->i_sequence, 0, new);
-		if (old)
+		old = 0;
+		if (!atomic64_try_cmpxchg_relaxed(&inode->i_sequence,
+						  &old, new))
 			return old;
 		return new;
 	}
-- 
2.46.2